Establishing a Temperature Gradient to Enhance Geothermal Heating and Cooling of a High Tunnel

Progress report for FNC21-1278

Project Type: Farmer/Rancher
Funds awarded in 2021: $6,555.00
Projected End Date: 07/31/2023
Grant Recipient: Jellum Farm
Region: North Central
State: Iowa
Project Coordinator:
Eric Jellum
Jellum Farm
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Project Information

Description of operation:

Eric Jellum: I began farming in 1998 after moving to Iowa from Washington State. Prior to moving here, I worked for Washington State University for 20 years at an agricultural research station in Puyallup (near Seattle). Our research centered mainly on nitrogen, phosphorus, and heavy metal chemistry in soil, as well as cover cropping. During that time I got my Masters in Soil Science from Washington State University. I currently farm 160 acres in North Central Iowa together with my brother. We use strip tillage/no tillage, intercropping, and cover cropping to grow mainly corn and soybean in rotation.

Steve Strasheim: I have been farming since 2014, when I founded Twisted River Farm. Prior to that, I spent my career working in sales, marketing and management positions. Twisted River Farm operates a compact, one acre, greens-focused “urban style” farm in North Central Iowa, delivering year round produce to dozens of accounts across the region including grocery stores, restaurants, farmers markets and a robust home delivery service.


The primary benefits of this project will focus on improved income and profitability and improved market opportunity for local production of primarily leafy greens in high tunnels. Our strategy to achieve this end is through better control of temperature and humidity throughout the year without using expensive and fossil fuel energy intensive heat sources. The objective is to use buried water pipe to store or extract ground heat beneath a high tunnel greenhouse and water-to-air heat exchangers at the surface to convey heat to the growing space. Periodic temperature fluctuations due to changing ambient conditions including night temperatures dropping as low as -30 degrees F in northern Iowa necessitate supplemental heat or adequate heat storage and heat recovery capacity. Especially in northern climates, supplemental heat can be prohibitively expensive during the heart of winter. Reversing water flow direction for ground heat extraction from the direction used for heat storage should create a beneficial temperature gradient. Enhancing the capacity to use ground storage to buffer daily and periodic temperature and humidity swings and attain higher winter night temperatures is especially important when potentially lethal cold temperatures can occur.

Project Objectives:

The objectives are to: 1) Adequately size, creatively design, and effectively operate a system to convey heat to and from the ground beneath a high tunnel to the growing space using water pipe for the ground loop and water-to-air heat exchangers in the growing space. 2) Measure the magnitude of the heat gradient created by reversing water flow direction during storage and extraction of heat in the ground loop and the effectiveness of that gradient to buffer temperature and humidity. 3) To convey the experience and lessons learned to the public and other growers in meetings, articles, and social media.


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  • Steve Strasheim - Producer


Materials and methods:

Prior to erecting a 30’ by 50’ high tunnel, foam insulation board will be buried 4’ deep under the high tunnel perimeter. Water line (1200’ of 3/4”) will be buried 3’ to 6’ deep, arranged in an inner loop and an outer loop near the perimeter. After erecting the high tunnel, cool season greens will be planted. The ground loop will be filled with water (23% propylene glycol) and connected to a pump and heat exchangers/fan at the surface. The pump and a fan will convey heat between the ground and the growing space. Heating and cooling will be controlled by separate thermostats. Excess heat will be stored in the ground, venting outdoors only if necessary or desirable. A temperature gradient will be established in the ground loop by thermostatically controlling flow direction using electric solenoid valves, one direction for heat storage and the other for extraction. In this way, the ground is warmest where the heated water first enters the soil. By reversing the flow direction during extraction, the water will be heated more by passing through the warmest soil just before entering the heat exchanger. System improvements will result from comparison trials using varying combinations of components, including: number of heat exchangers; heat exchangers in series vs parallel; fan size; isolating portions of the ground loop; water flow direction and flow rate comparisons. A third heat exchanger will be dedicated to using well-water in an open loop (emptying into a drainage tile) as needed for additional air dehumidifying beyond what can be accomplished by the closed ground loop system, especially when ground temperatures become too warm in the summer or cold in the winter. Heat storage/extraction and electricity use, temperatures, and humidity will be monitored. Trials throughout the project will be results driven from earlier trials and experience.

The project was begun in early summer of 2021 and proceeded according to the proposal with few exceptions.

Water line and drain tile installation
Trenching for geothermal pipe installation

Site water-packed and ready to smooth
Site smoothed off
Greenhouse frame
Project plumbing and heat exchangers installed

The high tunnel dimensions were 30’ by 54’, and the combined geothermal loop length was 1300’. The loops were oriented vertically in a narrow trench in a slinky pattern. The outer loop trench was about 140’ long with 700’ of water pipe. The inner loop was about 100’ long with 600’ of water pipe. Purging the system of air proved to be quite difficult because of the vertical orientation of the slinky loop. The flow rate in the outer loop was much lower than that in the inner loop for reasons that could never be determined. A loop diameter of about 42” resulted in over 100 loops in the entire system, each with the potential for trapped air at the top of the loop to limit flow rate. The circulating pump selected for the system was based on experience with a closed loop designed for drying corn (FNC17-1080) that consisted of three ¾” pipes oriented horizontally and manifolded to 1¼” at the heat exchanger and pump. Because the flow rate in the high tunnel system was substantially lower than the drying loop when the same pump was used a larger pump was purchased to increase the flow rate enough to adequately test the system. In hindsight, if the geothermal pipe slinky system had been oriented horizontally as in the drying loop, and three loops used instead of two, the larger pump and greater electricity use would not have been necessary. The larger pump in the high tunnel operating on the highest setting (about 200 watts) gives comparable flow (4.2 gallons per minute) as the smaller pump in the corn drying loop is capable of with only 77 watts. Restrictions in the above ground plumbing fittings in the system have also been identified and will be changed to increase flow rate.

Up until mid-December air temperatures were unseasonably mild. Soil temperature was maintained at a sufficiently high level to provide adequate heat at night, and produce was being harvested as needed. The system was operated manually without thermostats and without reversing flow direction in order to get some baseline experience with the system performance. On December 15th the Upper Midwest experienced a Derecho (wind storm) that affected our farm severely. Included in the damage we experienced was a large section of metal-clad barn wall that flew into the high tunnel, slashing plastic and bending some framework.

Derecho damage
Barn wall in greenhouse

Through the remainder of the night high wind continued to expand the size of the torn area. The following day when the wind subsided, a group of helpful neighbors came to assist in putting a tarp over the damage before the approaching cold weather threatened the growing crop within. Although the tarp was quite effective, it was opaque and blocked sunlight from a third of the greenhouse space.

Damaged greenhouse tarped

Even though it was secured tightly with ropes wind was able to penetrate all too easily. This was quite a setback for the planned evaluations. The impact of colder weather on soil temperatures was more pronounced because of the damage. On January 13th a brief stretch of sufficiently mild temperatures and low wind speed presented an opportunity to repair the damage just in time for about 8” of snow the following day.

Greenhouse repaired January 13

January 30, 2023 This is a progress report rather than a final report. An extension was granted to submit the final report by the beginning of July due to unanticipated circumstances. Flying debris from the Derecho wind event in December of 2021 did extensive damage to the high tunnel. Damage was repaired initially only well enough to seal the worst of the air leaks with an opaque tarp covering a third of the roof for the next month before getting an opportunity to replace the tarp with new plastic. Data was collected through the winter, but under compromised conditions. Tattered and taped plastic, mainly on the end-wall, has been replaced with twin-wall polycarbonate sheets, which resulted in a much tighter seal from ambient air penetration. A second entire winter for data collection will be possible with the extension.

In addition to storm damage repair, plumbing fittings for the geothermal system were increased in size where flow was considered to be restricted. This resulted in greater flow rates for the same pump setting. A third loop was added in October, 2022 between the inner (600' vertical slinky) and outer (700' vertical slinky) loops that were originally installed. The third loop is 110' long and was buried only 24 to 30” deep, laying flat in the trench rather than in a slinky pattern. One-inch diameter geothermal pipe was used rather than 3/4” like the original loops in order to accommodate insertion of a 1/2” PEX pipe that could be used for circulating water from the well. This makes the third loop a double acting heat exchanger that can not only exchange heat with the surrounding soil but also with well water that can be circulated through the PEX inside the geothermal pipe. Trials will be conducted during the winter.

A larger geothermal loop (240' and 8' deep) that has been used to dry corn for six years (FNC17-1080) was near enough to be fairly easily extended to the greenhouse when the trenches were open to install the greenhouse geothermal system. Since this connection was an afterthought not in the original design, both incoming and outgoing pipes to this loop were laid somewhat haphazardly without separation in the same trench and in close proximity to each other. So some heat exchange in the trench moderates the temperature of the fluid reaching the heat exchanger in the greenhouse. Despite that limitation, the dryer loop has been used as backup to add heat to the greenhouse when the greenhouse system was anticipated to be inadequate during low temperatures to keep up with heating demands. There are two air-to-liquid heat exchangers in front of the fan in the greenhouse. One is connected to the geothermal system within the greenhouse. The other is connected to the dryer loop. One or both systems can be operated independently to supply heat. The pump for the greenhouse loop has been left running both day and night during December, so that excess heat can be stored in the ground during daylight hours when air temperatures in the greenhouse are often high.

Research results and discussion:

Temperature monitoring has continued through the entire period while relative equilibrium is reestablished, but soil temperatures have dropped significantly as of the time of this report. It is difficult to estimate how much of this has resulted from the damage to the greenhouse and the length of time that substantial reduction in solar gain and cold air penetration because of the tarp.

Graphed data is shown for the period following the repair (January 13th) to illustrate the ability of the greenhouse and heating system to buffer temperature fluctuations as compared to ambient air outside. Even when outside temperatures have been as low as -24 F, inside temperatures have remained above 8 F. Although this temperature is lethal to the plants within, temperature spreads from outside to inside of up to 30 degrees F and more are encouraging. The experience we are getting should help us to concentrate on improvements that can help us meet our goal of crop production through the winter with very little energy use for heating besides what is required to access and distribute the solar energy stored in the ground and incident on the greenhouse roof each day.

SARE graphs - results

In the spirit of research, Steve planted varieties in the hoophouse in the Fall of 2021 which otherwise wouldn't have been considered given the crops low tolerance for deep cold: Broccoli, cabbage, kohlrabi, and 5 different varieties of head lettuce. Results through January 2022 have not been favorable to those varieties. Though, there is some potential with the lettuces if internal temperatures can be held slightly higher than we were able to in the nights that dipped into the -10 to -20 F outside temperature range. The spinach that was planted from seed had poor germination due to high fall temperatures but has performed wonderfully overall with very little leaf quality deterioration. Crop plans have already been discussed for the 22-23 winter rotation based on our findings so far. 

In the second winter of the project, since the greenhouse has been growing marketable crops essentially continuously throughout the year, a strategy was adopted to evaluate the ability of the geothermal systems to meet heat demands without allowing lethally cold temperatures, if possible, and without supplying supplemental heat from a non-geothermal source. The greenhouse system alone has been relied on unless it became clear enough that it may not maintain temperatures above the critical threshold throughout the night. When each loop is isolated from the others, the three loops in the greenhouse system have flowrates that vary from less than 1 gpm in the outer slinky loop, 2.5 gpm in the inner slinky, to 4.1 gpm in the recently added horizontal loop. The new loop is one inch in diameter, but with the half inch PEX inside of it the remaining cross sectional area is about the same as that in the 3/4” pipes. The higher flow rate is likely due to the shorter length and the flatter installation of the pipe. Repeated efforts to drive air from the outer loop have failed to increase the flow rate. The flow rate for all three lines together is 5.1 gpm at the low pump setting (150 Watts). A thermometer in the manifold in which the three lines come out of the ground and another placed in the line after the fluid passes through the heat exchanger are used to measure heat removed from or deposited into the ground. Periodically, when heat is not required, the fan is turned off to prevent heat exchange in order to approximate average temperatures in the ground loops. Even when ground temperatures, as measured using fluid temperatures in the loops, are similar in the greenhouse and dryer systems, the greenhouse system is much less able to buffer diurnal temperature changes than is the dryer system, in which changes in fluid temperature are very small. Undoubtedly, this is due partially to the longer pipe length in the dryer system (2400') compared to the greenhouse system (1400'), but the magnitude of the temperature swings may also be due to drier soil around the pipes in the greenhouse decreasing thermal conductivity. Drip irrigation lines were placed above the loops in the greenhouse system during their installation, but the soil weight on top of the collapsable lines may be inhibiting water flow from thoroughly wetting the soil around the pipes. When falling greenhouse loop temperatures appear unable to moderate dropping night time temperatures sufficiently to prevent lethal temperatures to the plants, the dryer loop has been engaged to provide additional heat. Using heat delivered from both the greenhouse loop and the dryer loop has kept temperatures above the desired 20 degree threshold even at ambient temperatures as low as -12 degrees, the coldest so far this winter.

Although there have been satisfying accomplishments so far in the project, there are important questions remaining. At present, crop quality remains excellent. This has been done solely with geothermal heat without supplementation. The backup geothermal system that has been used for corn drying has been turned on frequently to keep from jeopardizing the crop. As a result, it will take longer to determine the heating capacity of the greenhouse system when operating by itself. Even if both systems operated together continuously throughout a 100-day period in the heart of winter, the cost for pump operation on the low setting for each would be less than $50. Fan operation for heat distribution would be nearly identical. Whether or not a smaller fan might accomplish heat distribution adequately should be explored. Clearly, the operating cost of a geothermal system could be dwarfed by the capital cost of installation. The questions concerning size and design of a system still need to be addressed. The larger and deeper dryer loop does a better job maintaining heating capacity than the greenhouse loop, but will also continue to slowly cool during the remainder of winter, since it is exposed to continually cold ambient temperatures. The greenhouse system, although smaller, is a combination of solar collector and geothermal heat storage. From this point in the winter solar energy and daylength will steadily increase. The stored solar energy will increase the heating capacity of the system. The greenhouse system may prove itself to be superior to the larger dryer loop late in the winter. Also, since geothermal systems are expensive to install, heat retention alternatives for greenhouse design and mechanisms to ease nightly thermal blanketing should be considered. Benefits from flow reversal still need to be determined.

There are also vulnerabilities that should be addressed. This has so far been overlooked because we are so seldom without power from the grid. The antifreeze concentration in the system (23%) is the same as that used in geothermal heating applications that are not ordinarily exposed to ambient air temperatuers. The above-ground exposure, especially in the small cavities of the heat exchanger, leave our system vulnerable to freezing during a power outage. The freezing point of propylene glycol at this concentration is 16 degrees. This is well above the burst potential of zero degrees, but could make it difficult to resume flow necessary for heat supply if the outage was long enough for ice to block the pores in the heat exchanger. A strong enough antifreeze solution to eliminate freezing concerns (50% in northern Iowa) would be expensive and a larger environmental concern if the entire volume in the system contained a stronger solution. Another way might be to leave the geothermal system safely underground, with a short loop from a liquid-to-liquid heat exchanger to the surface air-to-liquid heat exchanger that could contain a stronger antifreeze solution. This could protect the system, although not the plants. A backup power supply that would automatically start during a power outage would be required to address this concern.


Participation Summary
2 Farmers participating in research

Educational & Outreach Activities

5 Webinars / talks / presentations

Participation Summary:

Education/outreach description:

Steve has been a guest on five podcasts since Nov 2021 talking about his farm, but making sure to mention the project and it's importance to the future of the operation. One show in particular, The Shark Farmer podcast, which aired on SiriusXM ch 147 in November 2021 took particular interest in the project and spent a few minutes asking questions about it. 

Looking ahead a few months, Steve has also secured speaking opportunities at two separate events, one in the early spring and one in the late spring of 2022, to talk specifically about the project and our findings through the 1st winter of the study. 

The outreach portion of this project has largely been achieved through social media, the blog section of the website and through word of mouth at various conferences and community events. Since our data was compromised in the aforementioned storm damage, we felt that postponing a proposed field day to the spring of 2023 was appropriate. We’ll continue to post updates on our Social Media and Website as there is a lot of interest in what this system is able to do.

I’m also trying to arrange a few speaking slots at two 2024 winter conferences. From the agronomy perspective-This winter has been far more favorable to our crops than last year. Lettuce, which is our highest demand leafy green, is quite frost tolerant down to 22 degrees (Winter vegetable killing temperatures. Adapted from Pam Dawling, Storage of Vegetables for Off-Season Sales, 2017 ). Through careful variety selection we have found that lethal threshold to be closer to 12 degrees, with leaf quality staying close to pristine down to 20 degrees. So far this winter, we have been able to keep the ambient temperature right around that 20 degree threshold that we feel is ideal for keeping leaf quality in pristine condition. In a typical winter season we would hope for a pristine lettuce crop up until around Jan 1. Our lettuce crop as of this writing is still in top quality condition, and is extending our sales period far longer than we had hoped for that product. Spinach and kale have a much lower temperature threshold, and typically are not as susceptible to leaf quality issues, thus our spinach and kale crops look as expected.

Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the view of the U.S. Department of Agriculture or SARE.